Exercise can be pretty complicated, especially with a lot of misleading information going around. Fitness tech company Digifit decided to dispel a handful of common cardio workout myths and found that while you might burn more calories with a harder workout, a slower one will burn more fat:

The best method for burning fat at a higher percentage is a steady, consistent workout in Zone 2 (60-69% max heart rate), the fat burning zone. This zone uniquely targets fat because fat is a slow burning fuel, so if you do a long and less-intense workout, your body will target a higher amount of fat cells then carbohydrates. While you may burn more net calories in higher heart rate zones, you will burn the highest percent of fat calories in Zone 2.

Pushing yourself too hard too often isn't good for you anyhow, so it's a good idea to make those intense workouts the exception rather than the rule. For more heart training myth busting, check out the full post over on Digifit.